The present invention relates to an optical transmitter and an optical transmission system.
Single-mode optical fiber is widely used for long-haul transmission in telecommunication systems. The Telecommunication Standardization Sector of the International Telecommunications Union (ITU-T) has set standards for two types of single-mode optical fiber. One type is described in ITU-T recommendation G.652, entitled Characteristics of a Single-Mode Optical Fibre Cable. The other type is described in ITU-T recommendation G.653, entitled Characteristics of a Dispersion-Shifted Single-Mode Optical Fibre Cable. Dispersion refers to chromatic dispersion of light pulses due to the different propagation speeds of different wavelengths. G.652 fiber has a zero-dispersion wavelength in the 1.3-.mu.m wavelength band. The zero-dispersion wavelength of G.653 fiber is located in the 1.5-.mu.m wavelength band, which has minimum attenuation loss.
Although the optical signals transmitted through these optical fibers can be generated by on-off modulation of a laser light source, there is a problem of wavelength shift, referred to as chirp, when the laser is modulated in this way. This problem can be largely overcome by operating the laser light source at a constant output level, and modulating the laser light externally.
One type of external modulator comprises a Mach-Zehnder interferometer fabricated in a lithium-niobate substrate. This type of device, referred to as a Mach-Zehnder lithium-niobate modulator, or simply as a lithium-niobate modulator, is capable of operating at very high speeds with relatively little chirp, enabling high-bit-rate transmission over long distances. However, there is a problem in that the attenuation characteristic of a lithium-niobate modulator changes slowly over time, but this problem, known as DC drift, can be overcome with a feedback loop that adjusts the bias voltage applied to the modulator. Lithium-niobate modulators are currently used in, for example, optical transmission systems operating at ten gigabits per second (10 Gb/s).
Although a lithium-niobate modulator has low chirp, the chirp is not necessarily zero, and zero chirp is not necessarily desirable. When optical signals with a center wavelength of 1.5 .mu.m are transmitted through G.652 fiber, for example, the dispersion penalty can be reduced if the optical modulator imparts a negative or blue chirp to the transmitted light pulses. When the same optical signals are transmitted through G.653 fiber, a small positive or red chirp may slightly reduce the dispersion penalty.
Conversely, light pulses with a 1.5-.mu.m center wavelength and positive chirp are severely broadened by transmission through G.652 fiber, while light pulses with a 1.5-.mu.m center wavelength and negative chirp are overly compressed by transmission through G.653 fiber. To avoid these types of signal distortion, it would be convenient if an optical modulator could be switched between positive and negative chirp, depending on the type of optical fiber employed and the transmission wavelength, but conventional lithium-niobate modulators provide no way of switching the chirp polarity.